Dynamic memory stores information as charge in a memory capacitor. In embedded memory (memory included in an integrated circuit with logic circuits, usually for storing data for specific logic functions) the memory capacitor is often implemented as a field effect transistor, the control electrode of this transistor forming one terminal of the capacitor and the drain and source of this transistor forming the other terminal of the capacitor.
Charge is supplied to the capacitor via an access transistor. When information is written the access transistor connects one of the terminals to the positive supply voltage or to the negative supply voltage, dependent on the information value that must be written. The other terminal of the capacitor is kept in connection with a plate conductor, which is at a substantially fixed voltage level.
The retention time of this information is limited by leakage currents, which over time efface the difference between charges that are used to represent different information values. The retention time can be improved by increasing the charge difference between the different charges that represent different information values.
The charge difference is limited amongst others by a threshold voltage drop across the access transistor. In case a field effect transistor is used as capacitor, the charge difference is also limited the minimum charge that is required to keep the channel of this field effect transistor conductive.
It is known to boost the voltage at the plate conductor so that the field effect transistor operates as a capacitor over a wider voltage range, thus increasing the charge difference. It is also known to boost a voltage at the control electrode of the access transistor outside a supply voltage range of the memory. This has the effect of overcoming the threshold voltage drop, thus also increasing the charge difference.
Boosting the voltage at the control electrode of the access transistor requires a strong and fast access transistor control circuit. The speed with which the control electrode can be driven strongly influences the access speed of the memory. Usually the control electrodes of a row of access transistors in a memory matrix are driven in parallel. This means that the access transistor control circuit must be able to drive the control electrodes of a row of access transistors quickly. As a result, the access transistor control circuit may be quite large.